Change List for Qt6-only based compilation from current master

[anytizer]

full log here, hope, it helps.

Major seeings:

• Only Qt6 was installed on the machine to compile it.
• References to Qt"5" replaced with Qt"6", particularly in cmake text files.
• Some QKeyEvent variables were recreated to their full forms.
• qreal, replaced with float. Some functions had ambiguity, and replaced by casting the input parameters.
• FileRevealer using QFileInfo instead of QString.
• QStringLiteral chained arguments. | could introduce a bug, not tested.
• layout->setMargin() - removed lines for window/widget layouts.
• Replaced some missing headers.

Post findings in the new version:

• zoom control with ctrl+mouse-wheel does not work on song editor when the mouse is inside the tracks boundary.
• some extra debug info on console.

[Rossmaxx]

https://github.com/LMMS/lmms/tree/qt6

[anytizer]

I had to disable VST due to it tried to use x11extras, which is removed. -DWANT_VST=OFF. Referred by src/3rdparty/qt5-x11embed.